According to a report at the National Meeting of the American Chemical Society, grapefruit, orange, and apple juice may reduce the body's absorption of certain medications you take by mouth.

Researchers from the University of Western Ontario in London and their colleagues showed that taking certain pills, such as the antihistamine fexofenadine and the antibiotic ciprofloxacin, with grapefruit juice decreased the absorption of the drug by half. They found similar results with orange and apple juice. They also found that taking the pills with water containing the grapefruit flavanoid naringin reduced drug absorption as well.

These same researchers had previously shown that grapefruit juice could boost concentrations of some drugs to potentially toxic levels in the blood. Given the unpredictable effect of grapefruit and other juices on medication levels in the bloodstream, it's important for patients to ask their health care providers about these potential concerns.